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EP3307921B1 - Hochfester stahl und herstellungsverfahren - Google Patents

Hochfester stahl und herstellungsverfahren Download PDF

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Publication number
EP3307921B1
EP3307921B1 EP16806952.4A EP16806952A EP3307921B1 EP 3307921 B1 EP3307921 B1 EP 3307921B1 EP 16806952 A EP16806952 A EP 16806952A EP 3307921 B1 EP3307921 B1 EP 3307921B1
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EP
European Patent Office
Prior art keywords
temperature
sheet
martensite
less
rolled
Prior art date
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Active
Application number
EP16806952.4A
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English (en)
French (fr)
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EP3307921A2 (de
Inventor
Josee Drillet
Veronique Hebert
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
ArcelorMittal SA
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ArcelorMittal SA
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Publication of EP3307921A2 publication Critical patent/EP3307921A2/de
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0436—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
    • C23C14/02—Pretreatment of the material to be coated
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C2/06—Zinc or cadmium or alloys based thereon
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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets

Definitions

  • the present invention relates to a cold-rolled and annealed steel sheet having very high mechanical strength and deformation capacity for the manufacture of parts by shaping, in particular in the automotive industry, for the manufacture of structural elements of automobile bodies, and the manufacture of such a sheet.
  • steels whose structure includes martensite, possibly bainite, within a ferritic matrix, have undergone great development because they combine high resistance with significant deformation possibilities.
  • these steels must have good ductility, good weldability and good coating suitability, in particular good suitability for continuous hot-dip galvanizing.
  • These steels must also have a high yield strength and elongation at break as well as good formability.
  • Some automotive parts are manufactured by forming operations combining different deformation modes. Some microstructural characteristics of the steel may be well suited to one deformation mode, but unfavorable to another mode. Some parts of the parts must have high resistance to elongation and/or good bending ability and/or good ability to form a cut edge, particularly when forming with embossed edges.
  • This formability of a cut edge is assessed by determining a hole expansion ratio, denoted Ac%. This ratio measures the steel's ability to undergo expansion during cold stamping and therefore provides an assessment of the formability in this deformation mode.
  • the hole expansion ratio can be evaluated as follows: after making a hole by cutting in a sheet metal, a truncated cone-shaped tool is used to expand the edges of the hole. It is during this operation that early damage can be observed near the edges of the hole during expansion, this damage initiating on second-phase particles or at the interfaces between the different microstructural constituents in the steel.
  • EP2426230 is a document which has high strength galvanized steel sheet exhibiting excellent forming, welding and fatigue resistance properties, with a composition consisting of C: 0.05% to ⁇ 0.12%, Si: 0.35% to ⁇ 0.80%, Mn: 2.0 to 3.5%, P: 0.001 to 0.040%, S: 0.0001 to 0.0050%, Al: 0.005 to 0.1%, N: 0.0001 to 0.0060%, Cr: 0.01% to 0.5%, Ti: 0.10 to 0.080%, Nb: 0.010 to 0.080%, B: 0.0001 to 0.0030%, optionally Mo: 0.01 to 0.15%, Ca: 0.0001 to 0.0050%, rare earths: 0.0001 to 0.1%, Sb: 0.0001 to 0.1%, and possibly V: less than 0.05%, Zr: less than 0.0200%, Mg: less than 0.0200%, Cu: less than 0.4%, and Ni: less than 0.4%, always in mass percentage, the remainder being Fe and unavoidable impurities;
  • EP2578718 is a document which has high strength galvanized steel sheet having excellent bending and welding properties, made, in which a surface layer of the steel sheet, which constitutes a portion of the steel sheet up to a depth of 10 mm measured from each surface of the steel sheet, has a structure containing more than 70% ferrite phase by volume fraction, an inner layer portion of the steel sheet, on the side further inward than the depth of 10 mm measured from each surface, has a structure consisting of 20 to 70% by volume fraction of ferrite phase with an average crystal grain size of 5 mm or less; 30 to 80% bainite phase and/or martensite phase by volume fraction, with an average crystal grain size of 5 mm or less; and a residual austenite phase and/or a pearlite phase less than or equal to 5% (0% inclusive) in volume fraction as the remainder, the steel sheet has a tensile strength greater than or equal to 980 MPa and the steel sheet has a galvanized layer on one of its surfaces.
  • EP2123786 is an invention relating to a cold-rolled and annealed Dual Phase steel sheet with a strength of between 980 and 1100 MPa, with an elongation at break of greater than 9%, the composition of which comprises, the contents being expressed by weight: 0.055% ⁇ C ⁇ 0.095%, 2% ⁇ Mn ⁇ 2.6%, 0.005% ⁇ Si ⁇ 0.35%, S ⁇ 0.005%, P ⁇ 0.050%, 0.1 ⁇ Al ⁇ 0.3%, 0.05%:5Mo ⁇ 0.25%, 0.2% ⁇ Cr ⁇ 0.5%, it being understood that Cr+2Mo ⁇ 0.6%, Ni ⁇ 0.1%, 0.010 ⁇ Nb ⁇ 0.040%, 0.010 ⁇ Ti ⁇ 0.050%, 0.0005 ⁇ B ⁇ 0.0025%, 0.002% ⁇ N ⁇ 0.007%, the remainder of the composition being made up of iron and unavoidable impurities resulting from the production.
  • an aim of the invention is to provide a steel sheet having a high mechanical strength, in particular between 1180 and 1320 MPa, together with a high elastic limit, in particular between 750 and 970 MPa, this value being determined before any skin-pass operation on the sheet, good formability, in particular a hole expansion ratio Ac% greater than or equal to 20%, a bending angle, for a sheet with a thickness of between 0.7 mm and 2 mm, greater than or equal to 40°, and an elongation at break greater than 7%.
  • the invention relates to a cold-rolled and annealed steel sheet with a thickness of between 0.7 mm and 2 mm, with a mechanical strength of between 1180 MPa and 1320 MPa, the hole expansion ratio Ac% of which is greater than 20% and the bending angle greater than or equal to 40°, the chemical composition of which comprises, the contents being expressed by weight: 0.09 ⁇ C ⁇ 0.11%, 2.6 ⁇ Mn ⁇ 2.8%, 0.20 ⁇ Si ⁇ 0.55%, 0.25 ⁇ Cr ⁇ 0.5%, 0.025 ⁇ Ti ⁇ 0.040%, 0.0015 ⁇ B ⁇ 0.0025%, 0.005 ⁇ Al ⁇ 0.18%, 0.08 ⁇ Mo ⁇ 0.15%, 0.020 ⁇ Nb ⁇ 0.040%, 0.002 ⁇ N ⁇ 0.007%, 0.0005% ⁇ S ⁇ 0.005%, 0.001% ⁇ P ⁇ 0.020%, Ca ⁇ 0.003%, the remainder consisting of iron and unavoidable impurities from the production
  • Ar3 will denote the temperature at which the austenite transformation begins upon cooling.
  • Ms will denote the temperature at which martensitic transformation begins.
  • the microstructure of the sheet according to the invention comprises martensite. This results from a diffusion-free transformation of ⁇ austenite below the martensitic transformation start temperature Ms upon cooling.
  • Martensite occurs as thin, elongated laths in one direction, oriented within each initial austenite grain.
  • the term martensite includes both fresh and self-tempered martensite. Self-tempered martensite will subsequently be distinguished from fresh, i.e., untempered and non-self-tempered martensite.
  • self-tempered martensite occurs in the form of thin laths comprising iron carbides dispersed in these laths, in the form of rods oriented along the ⁇ 111> directions of the ⁇ ' lattice of the laths.
  • This self-tempered martensite is formed by precipitation of iron carbides below the martensitic transformation temperature Ms when cooling is not slow enough to produce fresh martensite.
  • fresh martensite does not comprise carbides.
  • Bainite formed during cooling from the austenitic region above the martensitic transformation onset temperature Ms, occurs as an aggregate of ferrite laths and cementite particles. Its formation involves short-range diffusion.
  • Lower bainite is formed, during cooling, in a range of temperatures immediately above the martensitic transformation temperature Ms. It occurs in the form of thin laths and includes carbides dispersed in these laths.
  • low carbide bainite is defined as bainite containing less than 100 carbides per 100 square micrometers of surface area. Low carbide bainite is formed during cooling between 550°C and 450°C. Unlike low carbide bainite, lower bainite always contains more than 100 carbides per 100 square micrometers of surface area.
  • carbon plays a role in the formation of the microstructure and in the mechanical properties.
  • the carbon content by weight is between 0.09% and 0.11%. This carbon content range contributes to simultaneously obtaining a mechanical strength greater than 1180 MPa, an elongation at break greater than 7%, and a satisfactory hole expansion ratio Ac%, greater than or equal to 20%. In particular, a carbon content lower than 0.09% does not allow sufficient mechanical strength to be achieved. For a higher carbon content, greater than 0.11%, the weldability tends to decrease and the Ms temperature is lowered, so that the fraction of fresh martensite in the microstructure tends to increase and thus degrade the hole expansion ratio.
  • the manganese content by weight is between 2.6% and 2.8%.
  • Manganese is a gammagenic element that lowers the Ac3 temperature and the Ms temperature at which martensite begins to form.
  • the low carbon content of the steel could lead to a high Ac3 temperature, above 860°C.
  • a manganese content above 2.6% allows, by lowering the Ac3 temperature value, to obtain complete austenization of the steel between approximately 840°C and 855°C, after maintaining it at this temperature for at least 30s.
  • Manganese also allows the formation of self-tempered martensite and therefore contributes to obtaining a hole expansion ratio Ac% greater than or equal to 20%.
  • the manganese content by weight is limited to 2.8% in order to limit the formation of banded structures, and preferably between 2.6 and 2.7%.
  • Silicon is an element involved in solid solution hardening, the content of which in the steel is between 0.20% and 0.55% by weight, preferably between 0.30% and 0.5%.
  • a content of at least 0.30% allows sufficient hardening of ferrite and/or bainite to be achieved.
  • the silicon content by weight is limited to 0.55% to ensure a hole expansion ratio Ac% greater than or equal to 20%, limiting the formation of higher bainite.
  • an increase in the silicon content degrades the coating ability of steel by promoting the formation of oxides adhering to the surface of the sheet.
  • a silicon content of less than 0.55% also helps ensure good weldability.
  • Silicon is an alphagenic element and helps raise the Ac3 temperature and promote the formation of low-carbide bainite.
  • a silicon content of less than 0.55% thus helps prevent the formation of too much low-carbide bainite.
  • the composition of the steel sheet also includes chromium in an amount greater than or equal to 0.25% by weight, so as to improve the hardenability of the steel, and to increase its hardness as well as its mechanical resistance.
  • the chromium content must be less than 0.5%, so as to maintain a satisfactory elongation at break and to limit manufacturing costs.
  • Titanium is present in steel in quantities between 0.025% and 0.040% by weight. In quantities between 0.025 and 0.040%, titanium combines in particular with nitrogen and carbon to precipitate in the form of nitrides and/or carbonitrides. Below 0.025%, there is a risk that the mechanical strength of 1180 MPa will not be achieved.
  • Titanium also ensures that the nitrogen is fully combined in the form of a hardening precipitation, so that the boron is in free form and can play an effective role in hardenability. Titanium is in a superstoichiometric quantity compared to nitrogen, so that the ratio: Ti/N is greater than 3.42.
  • the boron content by weight is between 0.0015% and 0.0025%.
  • boron makes it possible to control and limit diffusive phase transformations (ferritic or pearlitic transformation during cooling) and to form hardening phases (bainite or martensite) necessary to obtain high mechanical strength characteristics.
  • the addition of boron also makes it possible to limit the addition of hardening elements such as Mn, Mo, Cr and to reduce the analytical cost of the steel grade.
  • the minimum boron content to ensure effective hardenability is 0.0015%. Above 0.0025%, the effect on hardenability is saturated and a detrimental effect on coatability and ductility is observed.
  • the composition of the steel sheet also includes molybdenum, in an amount between 0.08 and 0.15% by weight. Molybdenum, like chromium, plays an effective role in hardenability, in a content greater than 0.08%.
  • a molybdenum content greater than 0.15% delays the recrystallization of ferrite.
  • the mechanical strength Rm is then too high, greater than 1320 MPa, which jointly results in a reduction in ductility.
  • an addition under the conditions of the invention makes it possible to obtain, in combination with titanium and niobium, a nanometric precipitation of molybdenum, titanium and Niobium carbonitrides (Ti, Nb, Mo)(C,N) which contributes to hardening and makes it possible to obtain a resistance Rm greater than or equal to 1180 MPa after annealing.
  • the chemical composition of hot-rolled steel sheet includes niobium, with a weight content between 0.020 and 0.040%. In quantities greater than 0.020%, niobium allows an increase in the strength Rm. Above a weight content of 0.040%, the recrystallization of austenite is delayed. The structure then contains a significant fraction of elongated grains, which no longer allows the desired hole expansion ratio Ac% to be achieved.
  • the nitrogen content by weight is between 0.002% and 0.007%.
  • the nitrogen content In order to form a sufficient amount of nitrides and carbonitrides, the nitrogen content must be greater than 0.002%.
  • the nitrogen content must be less than 0.007% in order to avoid precipitation of boron nitrides, which would decrease the amount of free boron.
  • An aluminum content by weight of between 0.005% and 0.18% ensures the deoxidation of the steel during its manufacture.
  • An aluminum content must be less than 0.18%, or even less than 0.030% to avoid an increase in the Ac3 temperature and avoid the formation of ferrite during cooling.
  • the sulfur content must be less than 0.005%. Beyond this content, ductility is reduced due to the excessive presence of sulfides such as MnS which reduce the deformation ability, in particular the hole expansion ratio Ac%. Obtaining extremely low sulfur contents, less than 0.0005%, is however very expensive, without significant advantage in terms of manufacturing costs. From a practical point of view, a sulfur content not lower than 0.0005% can therefore be retained.
  • the phosphorus content should be less than 0.020%. This is because phosphorus is an element that imparts solid solution hardening but reduces spot weldability and hot ductility, particularly due to its ability to segregate at grain boundaries or co-segregate with manganese.
  • the sum of the surface proportions of martensite and lower bainite is between 40 and 70%.
  • the microstructure also contains, in surface proportion, from 15 to 45% of bainite with a low quantity of carbides, from 5 to less than 20% of ferrite, and less than 5% of residual austenite in the form of islands.
  • the surface fraction of non-recrystallized ferrite relative to the total ferrite fraction is less than 15%, which makes it possible to simultaneously obtain a resistance Rm of between 1180 and 1320 MPa, an elongation greater than 7% and a hole expansion ratio greater than or equal to 20%.
  • the martensite is formed in particular from self-tempered martensite, the surface percentage of the sum of the self-tempered martensite and the lower bainite representing at least 5% of the entire microstructure, this proportion being able to go up to 50%.
  • Self-tempered martensite and lower bainite occur as thin laths, and include carbides dispersed within these laths.
  • self-tempered martensite and lower bainite include iron carbides Fe 2 C and Fe 3 C in the form of rods oriented in the ⁇ 111> directions of the lattice of the martensitic and bainitic laths.
  • the percentages of self-tempered martensite and lower bainite are specified jointly because self-tempered martensite and lower bainite have essentially the same role on the use properties of the steel.
  • these two constituents, present in the form of thin laths cannot be individually identified from each other during scanning electron microscopy observations. Only transmission electron microscopy examinations can distinguish these two constituents.
  • a surface percentage of the sum of self-tempered martensite and lower bainite of between 40% and 70% helps to promote the formability of the steel, in particular its bendability and its ability to form a cut edge.
  • a percentage of self-tempered martensite and lower bainite of at least 40% thus contributes to obtaining a satisfactory bending angle, in particular a bending angle for sheets with a thickness of between 0.7 mm and 2 mm, of at least 40°, and a hole expansion ratio Ac% greater than or equal to 20%.
  • the percentage of the sum of self-tempered martensite and lower bainite in the microstructure must be less than 70% in order to maintain a sufficient percentage of bainite with a low quantity of carbides, allowing an elongation at break of at least 7% to be obtained.
  • Martensite may also include fresh martensite in a surface percentage of between 15 and 45% of the entire microstructure.
  • the surface percentage of fresh martensite must be less than 45%, in particular to avoid reducing the ductility of the steel and to ensure a good hole expansion ratio.
  • the microstructure also comprises, in surface proportion, 15 to 45% bainite with a low quantity of carbides. This is formed during cooling after annealing at temperature Tm, and during maintenance between 550°C and 450°C. Its formation is favored in particular by the addition of silicon which tends to delay the precipitation of carbides, together with a small quantity of quenching elements such as carbon or manganese.
  • Low carbide bainite increases the elongation at break.
  • a low carbide bainite surface percentage of at least 15% allows an elongation at break of at least 7% to be obtained.
  • the low carbide bainite surface percentage must be limited to 45% to ensure a hole expansion ratio greater than or equal to 20% and a mechanical strength greater than or equal to 1180 MPa.
  • the microstructure also contains 5 to less than 20% ferrite in surface proportion. If the ferrite content is less than 5%, there is a risk of obtaining too much self-tempered martensite combined with too low an elastic limit. If the ferrite content exceeds 20%, there is a risk that the resistance Rm will be less than 1320 MPa.
  • the microstructure may also contain up to 5%, in surface proportion, of residual austenite in the form of islands, notably forming platelets between the laths of self-tempered martensite and lower bainite.
  • the inventors have also highlighted the importance of controlling the size of the austenitic grains created during the annealing of the cold-rolled sheet, i.e. existing at high temperature after the annealing, before subsequent cooling.
  • These austenitic grains are referred to as "old austenitic grains", because these grains are replaced by other constituents during the allotropic transformations during cooling.
  • the size of these old austenitic grains can nevertheless be demonstrated by different methods, on the final product.
  • the fraction of old austenitic grains whose size is less than one micrometer represents between 40 and 60% of the total population of these old austenitic grains
  • the fraction of old austenitic grains whose size is less than one micrometer is for example determined by means of a suitable reagent, the attack speed of which depends on certain local segregations at the old joints, such as for example the Béchet-Beaujard reagent, known in itself.
  • a sample of steel in the final state that is to say at the end of the manufacturing process according to the invention, is attacked by a suitable reagent, in particular a reagent composed of a saturated aqueous solution of picric acid with the addition of at least 0.5% of sodium alkylsulfonate, for a period of between a few minutes and one hour.
  • a micrographic examination of the sample makes it possible to visualize the joints of the old austenitic grains, and to produce a histogram of the distribution of the size of these old austenitic grains, in particular to determine the fraction of old austenitic grains whose size is less than one micrometer.
  • the size of the old austenitic grains can be determined by interrupted quenching during cooling after annealing, adopting initial cooling conditions so as to induce intergranular ferritic nucleation and then interrupting this by quenching.
  • the inventors have demonstrated that the size of these old austenite grains conditions the kinetics of phase transformation during cooling following the annealing.
  • small austenitic grains less than one micrometer, contribute to lowering the Ms temperature value and thus increasing the formation of fresh martensite.
  • microstructural characteristics presented above are determined for example by observing the microstructure by Scanning Electron Microscopy using a field effect gun (SEM-FEG technique) at a magnification greater than 1200x, coupled with an EBSD detector (Electron Backscatter Diffraction).
  • SEM-FEG technique field effect gun
  • EBSD detector Electro Backscatter Diffraction
  • the cold-rolled and annealed steel sheet according to the invention may be produced bare, without coating, but it may also be coated.
  • a coating may be formed from zinc or a zinc alloy, in particular a galvanized-alloy coating comprising from 7 to 12% by weight of iron.
  • such a steel sheet is well suited to the deposition of a metallic coating, in particular by dip galvanizing using conventional processes.
  • the composition and mechanical characteristics of the steel are compatible with the stresses and thermal cycles of continuous dip zinc coating processes.
  • the coating process used depends on the intended application.
  • the coating can be obtained by dipping, by a vacuum deposition technique such as JVD (Jet Vapor Deposition), or by cationic electrodeposition.
  • a steel sheet according to the invention has a mechanical strength of between 1180 and 1320 MPa, together with an elastic limit of between 750 and 970 MPa, before any skin-pass operation, an elongation at break of at least 7%, in particular greater than 8%, and a hole expansion ratio Ac% greater than or equal to 20%.
  • a yield strength of between 800 and 970 MPa is obtained while maintaining a mechanical strength of less than 1320 MPa.
  • such sheet metal has a high bending angle.
  • the bending angle is at least 40°.
  • the implementation of the method for manufacturing a rolled sheet according to the invention comprises the following successive steps: A steel of the composition according to the invention is supplied, and a semi-finished product is cast from this steel. This casting can be carried out in ingots or continuously in the form of slabs with a thickness of around 200 mm.
  • the cast semi-finished products are first brought to a temperature T R above 1250°C, in order to homogenize the steel and completely dissolve the precipitates.
  • the semi-finished product is hot rolled in a temperature range where the structure of the steel is completely austenitic, i.e. at a temperature T FL higher than the Ar3 temperature at which the austenite transformation begins on cooling. If the temperature T FL is lower than the Ar3 temperature, the ferrite grains are work-hardened by rolling and the ductility is reduced. Preferably, an end-of-rolling temperature higher than 875°C will be chosen.
  • the hot-rolled sheet is cooled at a rate exceeding 30°C/s to prevent the formation of ferrite and pearlite, and then the hot-rolled sheet is coiled at a temperature T Bob between 500°C and 580°C.
  • the coiling temperature must be below 580°C to prevent oxidation during coiling. Too low a coiling temperature, i.e. below 500°C, leads to an increase in the hardness of the steel, which increases the forces required during subsequent cold rolling.
  • the coiling temperature range also prevents the formation of pearlite.
  • the sheet metal is then stripped, using one of the processes known in themselves.
  • Cold rolling is then carried out, with a reduction rate of, for example, between 40% and 70% so as to introduce a quantity of deformation allowing subsequent recrystallization.
  • the cold-rolled sheet is then heated, preferably in a continuous annealing installation, with an average heating rate V C of between 1°C/s and 20°C/s between 600°C and the temperature Ac1 (temperature at which allotropic transformation into austenite begins upon heating).
  • the temperature Ac1 is expressed in degrees Celsius, and the elemental contents of the composition are expressed in percentages by weight.
  • an average heating rate V C lower than 1°C/s would lead to too long a heating time between 600°C and Ac1, therefore to too much ferrite formation and too low a mechanical resistance.
  • the inventors have demonstrated that the size of the ferritic grains obtained after heating between 600°C and Ac1 has an influence on the size of the austenite grains after austenization. Insufficient growth of the ferritic grains in fact leads to the formation of too large a fraction of small austenite grains, and therefore insufficient formation of self-tempered martensite after annealing, i.e. less than 40%, due to the lowering of the temperature value M S .
  • the cold-rolled sheet is then heated from temperature Ac1 to an annealing temperature T m between 780°C and (Ac3-25°C)
  • the temperature Ac3 is expressed in degrees Celsius, and the elemental contents of the composition are expressed in percentages by weight.
  • the cold-rolled sheet is maintained at temperature T m for a duration Dm of between 30 and 150 seconds.
  • the duration Dm is chosen so that the fraction of austenitic grains whose size is less than one micrometer represents 40 to 60% of the total population of austenitic grains. A holding time Dm of less than 30s would lead to the formation of too large a proportion of ferrite at the end of the process.
  • the size of the austenitic grains determines the kinetics of phase transformation during cooling following annealing. In particular, small austenitic grains, less than one micrometer, contribute to lowering the value of the temperature Ms and thus to reducing the formation of self-tempered martensite.
  • Heating the cold-rolled sheet at an average heating rate V C of between 1°C/s and 20°C/s between 600°C and temperature Ac1, followed by heating the cold-rolled sheet between Ac1 and T m and maintaining the cold-rolled sheet at temperature Tm for a duration Dm of between 50 and 100 seconds thus makes it possible to control the size of the austenite grains formed, and more particularly to control the fraction of these grains whose size is less than one micrometer.
  • the steel sheet is cooled at a rate V R2 of between 10 and 100°C/s to a temperature Te of between 400°C and 490°C.
  • Cooling can be carried out from the temperature Tm , in one or more stages and can involve in the latter case different cooling modes such as cold or boiling water baths, water or gas jets.
  • the sheet is then maintained at temperature Te for a duration De of between 5 and 150 seconds.
  • a partial transformation of austenite into bainite with a low quantity of carbides occurs at this stage.
  • the maintenance at Te must be less than 150s in order to limit the surface proportion of bainite and thus obtain a sufficient proportion of martensite.
  • the following process steps differ depending on whether a continuously galvanized steel sheet, particularly a galvanized-alloyed one, is being produced, or an uncoated one.
  • the sheet is coated by continuous immersion in a zinc or zinc alloy bath at a temperature TZn of between 450°C and 480°C, for a few seconds.
  • the temperatures Te and TZn are such that 0°C ⁇ (Te-TZn) ⁇ 10°C.
  • the galvanized product is then cooled to room temperature, transforming a large fraction of the remaining austenite into fresh martensite and/or lower bainite.
  • the galvanized product is heated immediately upon leaving the zinc or zinc alloy bath to a temperature T G of between 490 and 550°C for a duration t G of between 10 and 40s. This causes the interdiffusion of iron and the thin layer of zinc or zinc alloy deposited during immersion, which makes it possible to obtain a galvanized-alloyed sheet.
  • the galvanized-alloyed sheet is then cooled to room temperature, transforming a large fraction of the remaining austenite into fresh martensite and/or lower bainite.
  • a cold-rolled and annealed steel sheet is thus obtained containing, in surface proportion, a sum of 40 to 70% of martensite and lower bainite, 15 to 45% of bainite with a low quantity of carbides, 5 to less than 20% of ferrite, and less than 5% of residual austenite in the form of islands.
  • the inventors have demonstrated that the implementation of this process makes it possible to obtain a steel sheet having a mechanical resistance of between 1180 and 1320 MPa, together with an elastic limit of between 750 and 970 MPa (before any skin-pass operation), an elongation at break of at least 7%, or even 8%, of which the hole expansion ratio Ac% is greater than or equal to 20%.
  • the implementation of this process makes it possible to ensure that the sheet has a bending angle of at least 40° when the sheet has a thickness between 0.7 mm and 2 mm.
  • the sheet obtained has good welding properties using standard assembly processes such as resistance spot welding.
  • steels I1 to I3 used for the manufacture of sheets according to the invention the composition of steels R1 to R5 used for the manufacture of reference sheets has been indicated for comparison purposes.
  • Cast semi-finished products corresponding to the above compositions were reheated to a temperature T R above 1250°C, then hot rolled, the end of rolling temperature being equal to 850°C, higher than Ar3 for all these steels.
  • the hot-rolled sheets were cooled to avoid the formation of ferrite and pearlite, and then coiled at a temperature of 545°C.
  • the sheets were then cold-rolled to a thickness of 1.4 mm.
  • I1 sheets were then heated with a heating rate V C , between 600°C and Ac1, Ac1 designating the temperature at which austenitic transformation begins upon heating, then from Ac1 to a temperature Tm and maintained at temperature Tm for a duration Dm.
  • V C heating rate
  • Ac1 designating the temperature at which austenitic transformation begins upon heating
  • the sheets were cooled at a rate V R1 to a temperature Te, then maintained at the temperature Te for a duration De.
  • sheet I1 was heated to 810°C, maintained for 120s at this temperature, then cooled by water quenching with a speed greater than 100°C/s to room temperature, i.e. without a temperature maintenance step Te.
  • grades R1-R5 were treated according to conditions defined in Table 3.
  • the mechanical characteristics of the sheets thus obtained, resulting from the steel composition and the heat treatment, were determined.
  • the sheets were identified by associating the name of the chemical composition and the heat treatment: thus I1A designates the sheet obtained from composition 11 to which the conditions of heat treatment A were applied.
  • the elastic limit Re By means of tensile tests, the elastic limit Re, the breaking strength Rm and the total elongation At of the sheets obtained by these different methods were determined. manufacturing methods. The bending ability of these sheets was also determined by determining the maximum angle before breaking.
  • This angle is determined by applying a punch to the sheets in order to bend them.
  • the force required to achieve the bend increases until it breaks. Measuring the force applied during bending thus makes it possible to detect the initiation of the sheet breaking, and to measure the maximum bending angle when this break occurs.
  • the hole expansion ratio Ac% of each sheet was also determined by cutting a 10 mm diameter hole in the sheet and then expanding the edges of the hole using a truncated tool. As described in ISO 16630:2009, the initial hole diameter Di before stamping was measured, followed by the final hole diameter Df after stamping, at the point where through-cracks in the sheet thickness were observed at the edges of the hole.
  • the microstructural characteristics of the steels were also determined.
  • the surface fractions of martensite (including self-tempered and fresh martensite) and lower bainite (jointly), self-tempered martensite and lower bainite (jointly), and low-carbide bainite were quantified on polished sections subjected to sodium bisulfite etching.
  • the surface fraction of fresh martensite was quantified after etching with a NaOH-NaNO3 reagent.
  • the surface fraction of ferrite was also determined using optical and scanning electron microscopy observations, where the ferritic phase was identified.
  • the mechanical properties of the sheets are shown in Table 5 below.
  • Table 5 Mechanical properties of the sheets obtained Re (MPa) Rm (MPa) At (%) Bending angle (°) Ac% I1A 793 1210 7.5 57 20 I1B 803 1166 8 nd 25 I1C 803 1253 9.5 60 nd I1D 621 1063 9.2 nd 25 I1E 636 1069 9.2 nd 25 I1F 1054 1370 6.7 nd 37 I1G 756 1237 10.1 nd 18 I1H 856 1327 7.5 nd 15.
  • the steel sheets according to the invention have a composition and a microstructure which make it possible to obtain a mechanical strength, an elastic limit, an elongation, a bending angle and a hole expansion ratio which satisfy the target values.
  • FIGS. 1 and 2 illustrate the microstructure of example I1A.
  • the Figure 1 shows the result of an attack on the sheet by sodium bisulfite, while the Figure 2 illustrates the result of the attack of the sheet by the reagent NaOH-NaNO3.
  • the Figure 1 reveals self-tempered martensite and lower bainite (M+Bl), as well as low carbide bainite (LCI).
  • Figure 2 reveals, in the form of darker zones, fresh martensite (MF).
  • the holding time Dm at temperature Tm of treatment G is too short so that the sheet produced according to condition I1G has a ferrite recrystallization rate of less than 15%. This generates a banded structure which leads to a low hole expansion value.
  • the too low annealing temperature leads to too high a density of small precipitates: the TEM observations ( Figure 4 ) show an average size of 5 nm in quantity greater than 10000 precipitates/ ⁇ m 3 so that the mechanical resistance exceeds 1320 MPa.
  • the low annealing temperature Tm does not allow a ferrite recrystallization rate greater than 15% to be reached, which leads to a hole expansion value that is too low.
  • the sheet according to example R1B has too high C, Cr, Ti and B contents, so that its mechanical strength Rm is too high despite too low a quantity of Mo. Thus, although a satisfactory yield strength Re is obtained, this yield strength is obtained in conjunction with too high a mechanical strength Rm.
  • the sheets according to tests R2B, R3B and R3C contain too low C, Mn and Mo contents, and thus do not have satisfactory mechanical strength although the quantities of B and Cr are very high.
  • Example R5 has too low a Mn content, which leads to too much bainite formation with low carbide content according to treatments B and C.
  • the steel sheets according to the invention will be used profitably for the manufacture of structural or safety parts in the automotive industry.

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Claims (11)

  1. Kaltgewalztes und geglühtes Stahlblech mit einer Stärke zwischen 0,7 mm und 2mm, einer Festigkeit zwischen 1180 MPa und 1320 MPa, dessen Lochausdehnungsverhältnis Ac% größer als 20 % und dessen Biegewinkel größer als oder gleich wie 40° ist, dessen chemische Zusammensetzung Folgendes umfasst, wobei der Gehalte nach Gewicht ausgedrückt ist: 0,09 ≤ C ≤ 0,11 %
    Figure imgb0032
     2,6 ≤ Mn ≤ 2,8 %
    Figure imgb0033
     0 , 20 ≤ Si ≤ 0 , 55 %
    Figure imgb0034
     0 , 25 ≤ Cr ≤ 0,5 %
    Figure imgb0035
     0,025 ≤ Ti ≤ 0,040 %
    Figure imgb0036
     0,0015 ≤ B ≤ 0,0025 %
    Figure imgb0037
     0,005 ≤ Al < 0,18 %
    Figure imgb0038
     0,08 ≤ Mo ≤ 0,15 %
    Figure imgb0039
     0,020 ≤ Nb ≤ 0,040 %
    Figure imgb0040
     0,002 ≤ N ≤ 0,007 %
    Figure imgb0041
     0 , 0005 % ≤ S ≤ 0 , 005 %
    Figure imgb0042
     0 , 001 % ≤ P ≤ 0 , 020 % ,
    Figure imgb0043
     Ca < 0 , 003 %
    Figure imgb0044
    wobei der Rest aus Eisen und unvermeidbaren Verunreinigungen aus der Verarbeitung besteht, wobei das Blech eine Mikrostruktur aufweist, die aus Martensit und/oder unterem Bainit besteht, der Martensit umfassend frischen Martensit und/oder selbsterzeugten Martensit, wobei die Summe der Flächenanteile, gemessen wie definiertin der Beschreibung, von Martensit und
    unterem Bainit zwischen 40 und 70 % liegt, von 15 bis 45 % Flächenanteil von Bainit mit geringer Karbidmenge, 5 bis weniger als 20 % Flächenanteil von Ferrit, wobei der Anteil von nicht rekristallisiertem Ferrit im Verhältnis zu dem gesamten Ferritanteil, weniger als 15 % ist, und weniger als 5 % Flächenanteil an inselförmigem Restaustenit, wobei die Fraktion der alten Austenitkörner mit einer Größe von weniger als einem Mikrometer 40 bis 60 % der Gesamtpopulation der alten Austenitkörner darstellt.
  2. Stahlblech nach Anspruch 1, dadurch gekennzeichnet, dass die Mikrostruktur in Flächenanteilen von 15 bis 45 % frischen Martensit enthält.
  3. Stahlblech nach Anspruch 1, dadurch gekennzeichnet, dass die Mikrostruktur in Flächenanteilen von 5 bis 50 % die Summe aus selbsterzeugtem Martensit und unterem Bainit umfasst.
  4. Stahlblech nach Anspruch 3, dadurch gekennzeichnet, dass der selbsterzeugte Martensit und der untere Bainit Karbide in Form von Stäbchen enthalten, die in den Richtungen <111> der martensitischen und bainitischen Latten ausgerichtet sind, wie sie gemäß der Beschreibung gemessen und identifiziert werden.
  5. Stahlblech nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, dass das Bainit mit geringer Karbidmenge weniger als 100 Karbide pro Flächeneinheit von 100 Quadratmikrometern enthält, gemessen wie definiert in der Beschreibung.
  6. Stahlblech nach einem der Merkmale 1 bis 5, dadurch gekennzeichnet, dass es Präzipitate vom Typ (Ti, Nb, Mo)(C,N) mit einer Größe von weniger als 5 Nanometern enthält, die in einer Menge von weniger als 10.000 Präzipitate / µm3 vorhanden sind.
  7. Stahlblech nach einem der Ansprüche 1 bis 6, dadurch gekennzeichnet, dass die chemische Zusammensetzung Folgendes umfasst, wobei der Gehalt in Gewicht ausgedrückt ist: 2 , 6 < Mn ≤ 2 , 7 % .
    Figure imgb0045
  8. Stahlblech nach einem der Ansprüche 1 bis 7, dadurch gekennzeichnet, dass die chemische Zusammensetzung Folgendes umfasst, wobei der Gehalt in Gewicht ausgedrückt ist: 0 , 30 < Si ≤ 0 , 5 % .
    Figure imgb0046
  9. Stahlblech nach einem der Ansprüche 1 bis 8, dadurch gekennzeichnet, dass die chemische Zusammensetzung Folgendes umfasst, wobei der Gehalt in Gewicht ausgedrückt ist: 0 , 005 < Al < 0 , 030 % .
    Figure imgb0047
  10. Verfahren zur Herstellung eines kaltgewalzten und geglühten Blechs nach einem der Ansprüche 1 bis 9, umfassend die folgenden aufeinanderfolgenden Schritte: - Bereitstellen eines Halbfabrikats, dessen chemische Zusammensetzung Folgendes umfasst, wobei der Gehalt in Gewicht ausgedrückt ist: 0 , 09 < C ≤ 0 , 11 %
    Figure imgb0048
     2 , 6 ≤ Mn < 2 , 8 %
    Figure imgb0049
     0 , 20 < Si ≤ 0 , 55 %
    Figure imgb0050
     0 , 25 ≤ Cr < 0 , 5 %
    Figure imgb0051
     0,025 < Ti < 0,040 %
    Figure imgb0052
     0,0015 ≤ B ≤ 0,0025 %
    Figure imgb0053
     0,005 ≤ Al ≤ 0,18 %
    Figure imgb0054
     0,08 ≤ Mo ≤ 0,15 %
    Figure imgb0055
     0,020 ≤ Nb < 0,040 %
    Figure imgb0056
     0,002 < N < 0,007 %
    Figure imgb0057
     0,0005 % ≤ S < 0,005 %
    Figure imgb0058
     0,001 % ≤ P < 0,020 % ,
    Figure imgb0059
     Ca ≤ 0,003 %
    Figure imgb0060
     wobei der Rest aus Eisen und unvermeidbaren Verunreinigungen besteht, die aus der Herstellung stammen, dann
    - Erhitzen des Halbfabrikats auf eine Temperatur Tr von 1250°C oder höher, dann
    - Warmwalzen des Halbfabrikats, wobei die Walzendtemperatur höher ist als die Temperatur Ar3 bei Beginn der Umwandlung des Austenits beim Abkühlen, um ein warmgewalztes Blech zu erlangen, dann
    - Abkühlen des warmgewalzten Blechs mit einer Geschwindigkeit von mehr als 30 °C/Sek., um die Bildung von Ferrit und Perlit zu verhindern, dann
    - Aufwickeln des warmgewalzten Blechs bei einer Temperatur zwischen 580 ° und 500 °C, dann
    - Kaltwalzen des warmgewalzten Blechs, um ein kaltgewalztes Blech zu erlangen, dann
    - Wiedererhitzen des kaltgewalzten Blechs zwischen 600°C und Ac1, wobei Ac1 die Temperatur bei Beginn der austenitischen Umwandlung beim Erhitzen bezeichnet, mit einer Erhitzungsgeschwindigkeit Vc zwischen 1 und 20 °C/Sek., dann
    - Bringen des kaltgewalzten Blechs auf eine Temperatur Tm zwischen 780 °C und (Ac3-25 °C) und Halten des kaltgewalzten Blechs über eine Dauer Dm zwischen 30 und 150 Sekunden auf dieser Temperatur Tm, wobei Ac3 die Endtemperatur der austenitischen Umwandlung beim Erhitzen bezeichnet, dann - Abkühlen des Blechs mit einer Geschwindigkeit VR1 zwischen 10 und 150 °C/Sek. auf eine Temperatur Te zwischen 400 und 490 °C, dann
    - Halten des Blechs auf der Temperatur Te über eine Dauer De zwischen 5 und 150 Sekunden, dann
    - Beschichten des Blechs durch kontinuierliches Eintauchen in ein Bad aus Zink oder einer Zinklegierung bei einer Temperatur TZn zwischen 450 °C und 480°C, wobei die Temperaturen Te und TZn derart sind, dass 0< (Te-TZn)≤10 °C, sodass ein beschichtetes Blech erlangt wird, dann
    - optional Wiedererhitzen des beschichteten Blechs auf eine Temperatur TG zwischen 490 °C und 550 °C über eine Dauer tG zwischen 10 Sek. und 40 Sek.
  11. Verwendung eines Stahlblechs nach einem der Ansprüche 1 bis 9 oder hergestellt durch ein Verfahren nach Anspruch 10 zur Herstellung von Struktur- oder Sicherheitsteilen für Kraftfahrzeuge.
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Designated state(s): AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR